Monitoring the mechanical integrity of pipelines, offshore production platforms, storage tanks and associated equipment is a complex problem. This is due to a wide variety of operational issues, especially those associated with the remote location of these structures. Current practice requires the dispatching of personnel to remote locations in order to conduct inspections of these assets using a variety of non-destructive inspection techniques. One particular area of integrity monitoring and measurement relates to the corrosion or loss of metal of these before-mentioned structures.
Measurement techniques commonly found in the industry include eddy current instruments, guided wave ultrasound, phased array ultrasound and direct ultrasound thickness measurements, to name a few. The use of ultrasound can be conveniently categorized as an ultrasound testing (UT) method. See Krautkramer, Josef and Krautkramer, Herbert, Ultrasonic Testing of Materials, 4th/revised edition, Springer Verlag, November 1990. In the case of pipeline corrosion monitoring there are typically three measurement techniques: (1) In-Line Inspection (ILI), (2) intrusive and (2) non-intrusive techniques.
In-Line Inspection (ILI) tools, or smart pigs, are intelligent sensing devices that are introduced into the line at a specialized entry point and most commonly conveyed by product flow along the length of the pipe. Although there are significant advantages to pipeline pigging, there are a number of operational complexities and limitations. In addition, this process is expensive and invasive to a pipelines operation. See Pipeline Pigging Handbook by Jim Cordell and Hershel Vanzant, CLARION Technical Publishers; 3RD edition, 2003.
Intrusive methods include electrical resistance probes and coupons. The coupon is the original form of intrusive corrosion monitoring. It can typically consist of a strip of metal made of material similar to the pipeline. It is weighed, then inserted it into an access point and left for several months for exposure to the product. The operator then removes the coupon and weighs it again to see what percentage is missing. As in any invasive technique, this approach to corrosion monitoring can be disruptive to pipeline operations. See Lawson, K. M., and N. G. Thompson. “The Use of Coupons for Monitoring the Cathodic Protection of Buried Structures.,” CORROSION/98, paper no. 672. Houston, Tex.: NACE, 1998.
The most common non-intrusive device is external monitoring such as eddy current testing and ultrasonic testing (UT) as cited earlier. In either case, in order to conduct a survey, a defined area is excavated around the pipeline and a portable device (pulser-receiver) is held against the pipe. Within the UT sensor, voltage is applied across a piezoelectric crystal to generate an ultrasonic sound wave that propagates through the metal. The time it takes to travel through the metal and back to the transducer is directly proportional to its thickness. The devices are quick, easy to use and inexpensive, and operators do not have to shut off flow or risk breeching the pipeline in order to take a reading. On the other hand, there are significant costs for digging up the pipeline each time a test is conducted. These labor costs are also high when the pipeline owner must dispatch inspectors to evaluate fixed locations on above ground pipelines, pipeline transitions and pipeline components. This is also the case for offshore pipeline structural components that are monitored on a routine basis both above and below the structure's waterline.
There are a number of other corrosion monitoring techniques available to the industry. Most, if not all, remote monitoring techniques use indirect measurement methods such as electrochemical potentials. See McMurry, John; Robert C. Fay (March 2004). “Electrochemistry”, Chemistry, 3rd Edition, Prentice Hall.
The problem with most measurement techniques is that:
1. The before mentioned measurement techniques require personnel to travel to asset location in order to take the measurement. Wireless networks exist for remote monitoring of flow, pressure, fugitive emissions and similar applications; however, most non-destructive measurements typically require the intervention of personnel in order to acquire the measurements;
2. Measurement tools typically require a power source that must be either available locally or accompanied with the person taking the measurement;
3. The location of many of these before-mentioned assets is difficult to access which poses safety risks to the inspectors;
4. Most permanent installations of remote measurement sensors are invasive and require some degree of operational downtime;
5. The environment in which measurements are taken can be potentially explosive; therefore, not all measurement techniques are possible without special intrinsically-safe equipment; and,
6. Mechanical degradation rates of some assets can be rapid, requiring frequent integrity assessment measurements, further increasing the personnel safety risk and operational cost.